Since an analog to digital converter ends up producing a number, wouldn't it be convenient if we could simply count something and have that something proportional to the potential being measured? A voltage to frequency converter ADC does just that. A voltage is integrated until it reaches some pre-set threshold, at which point a comparator trips, doing 2 things:

1) short circuit the capacitor, resetting the integrator to zero.

2) produce a pulse that is counted.

Here' the circuit.

V to F converters integrate noise, and so are useful under circumstances similar to dual slope units. V to F ADCs are precise, accurate, simple, and inexpensive. The precision is directly proportional to the time over which counting occurs, and inversely proportional to the time required to integrate a single count.